This invention relates generally to an apparatus and method for testing AC motor power systems and, more particularly, to an apparatus and method for nondestructive in-circuit testing for short-circuit conditions in an inverter power circuit.
It is common in AC electric motor drive systems to employ power inverter systems to furnish electric power from a DC source to an AC motor. These inverter systems are of various types, but are often comprised of gate turn-off thyristors (GTO) in a bridge arrangement. The GTOs in the bridge are selectively gated to control the electrical power supplied to the motor by converting DC power from the DC source into AC power which drives the motor. Typically, two GTOs are connected in a series arrangement in what is commonly referred to as a "leg" between relatively positive and relatively negative busses of the DC source. A common converter of this type is a three-phase converter having three legs connected in mutual parallel between the positive and negative DC source busses. The GTOs of each of the legs are rendered conductive in a predetermined order or sequence in order to control the electrical power delivered from the DC busses to the AC motor.
The above described motor systems require regular maintenance to avoid or detect a variety of common failure modes. For example, if both GTOs of a leg were to become conductive simultaneously, there would exist a-short between the DC source busses which, if allowed to continue, could result in great damage to the motor, power source, and/or to the GTOs. In addition, failure of various components and segments of the system such as the feedback circuits, drive circuits, and the motor load itself can occur. In prior art, expensive and time consuming manual off-line testing has been necessary to detect many errors at an early enough stage to ensure proper operation. In order to enhance maintenance of such systems, it would accordingly be highly desirable to provide an auto self test capability and, in particular, to provide a test capability to identify short-circuit conditions prior to initiating operation of an inverter.
The above as well as other desirable features are achieved in accordance with the present invention by testing a power inverter prior to enabling inverter operation. In an illustrative form, the power inverter has a plurality of legs having first and second series connected controllable switching devices, such as GTOs, connected between positive and negative DC voltage busses. The testing method includes measuring the voltage intermediate the switching devices prior to enabling the devices into conduction. The measured voltage is then compared to the voltage at each of the voltage busses. If the measured voltage is substantially equal to either one of the bus voltages, one of the devices is short-circuited and the system is inhibited from initiating operation of the inverter.
In a preferred form, the voltage intermediate the switching devices is measured a preselected time period after power is-applied to the voltage busses, the time period being selected to be sufficiently long to allow the voltage to settle and sufficiently short to prevent the voltage from discharging through the load or the measuring device.